According to the development of recent electronic apparatus, the automation of various machines and facilities has been advanced. Concomitantly, sensors of various kinds have become more necessary. For example, in an apparatus having an opening and closing member such as a door, cover and the like, the sensor is required for sensing an object or the hand of a human being caught into its opening when the opening and closing member is shut.
Previously, a sheet type of input switch or pressure sensor has widely been used, which is made by dispersing graphite or metal particles into silicone rubber to give conductivity and forming the mixture into a pressure sensitive and conductive rubber sheet. Such a prior art is disclosed in Japanese Patent Publication Nos. 40-24061; 57-53602; 56-54019; 58-24921; and Japanese Laid Open Patent Publication No. 53-897. Also, a cord-shaped switch or sensor having the long sheet sandwiched electrodes is described in Japanese Laid Open Patent Publication Nos. 61-161621; and 63-52024; and Rubber Industries, Vol. 21(1985), No.1.
Recently, a pressure sensor having a cavity between such conductive members to enhance a switching function and to ensure the ON/OFF operations is proposed in Japanese Laid Open Patent Publication No. 6-260054.
In recent years, to prevent an accident by which a part of the human body is caught by a window shield upon a motor-operated automatic opening and closing in an automobile, the development of a sensor to detect such a catch of the human body is urgently required. The use of such a prior sensor described in Japanese Laid Open Patent Publication Nos. 6-260054; 63-52024, etc. results in various problems in a sensing accuracy.
According to Japanese Laid Open Patent Publication No. 63-52024, a pressure is detected by the drop in electric resistance caused by pressurization, but charge of electric resistance is too low. In addition, the electric resistance is changed by internal stress generated within the sensor itself by bending thereof and the like, resulting in an erroneous operation of the sensor. According to Japanese Laid Open Patent Publication No. 6-260054, the disadvantage of the above low changed amount in electric resistance can be improved by providing a cavity between facing continuity members (electrodes), and detecting the pressure by means of contact between the continuity members caused by pressurization. However, this sensor has a serious defect in which the direction to be sensed is concentrated or biased in one direction, that is, it can not sense pressurization from the side. In addition, facing electrodes easily come into contact each other in a beat condition and thus, this sensor can not be used in a curved portion.
It is therefore an object of the present invention to provide a cord switch which can securely detect and carry ON/OFF operations, can cancel an erroneous operation by preventing contact between electrodes due to their bending, and have a positive sensitivity to pressurization in all directions, that is, a high reliability. Also, it is an object of the present invention to provide a pressure sensor which can extend the sensing range to the leading edge of the cord switch.